The present invention relates to glass manufacture and more particularly to a method for the manufacture of glass articles from glasses which are not stable against crystallization or devitrification when cooled from a melt.
It is well known that glasses vary widely in their tendency to devitrify on cooling from melts and/or on exposure to high temperature. Even for the manufacture of glass products from conventional glasses, which are generally formulated to maximize stability against devitrification, some attention must be given to the design of melting and forming processes to insure that devitrification of the glass during manufacture is avoided.
The process of devitrification is known to occur at a rate which is temperature dependent. Generally, crystals will not form rapidly in conventional glasses at temperatures below the glass transition temperature, nor will they form at all at temperatures above the liquidus temperature. The liquidus temperature is, by definition, the minimum temperature at which a crystal-free glass melt can be maintained indefinitely.
Within the range of temperatures below the liquidus temperature and above the glass transition temperature, the rate of crystallization can vary widely, depending on the temperature of the glass, the chemical composition of the glass, and the presence or absence of crystal nuclei therein. Some glass compositions crystallize extremely slowly, while other compositions crystallize so rapidly that they are difficult to form into crystal-free products.
The temperature regime of rapid crystallization of any glass composition can readily be determined, and glassforming processes adjusted to minimize the dwell time of the glass in that range. Thus, while some time in the crystallization range must always be tolerated, forming can be carried out such that the glass is rather rapidly cooled or quenched through that range and the development of crystals largely or entirely avoided.
Many compositions capable of forming stable glasses are known. The most widely used class of stable glasses comprises the silicate glasses, wherein SiO.sub.2 is a major or predominant constituent. However, relatively stable glass formulations have also been developed in systems based on phosphate, borate, and even halide or oxyhalide compositions.
Glasses formulated for high stability in any of these systems can generally be formed by conventional processes into crystal-free products. In most cases, they can also be reheated and worked or reshaped while softened without undergoing devitrification. In the most stable glassforming systems, the glasses can even be provided as powders, and sintered by heating below the liquidus temperatures into unitary glass articles which are completely amorphous, i.e., essentially free of crystalline inclusions or surface devitrification. Fused silica and doped fused silica are examples of glasses which can successfully be processed in this manner.
A very high degree of stability is essential for sintering processes of the latter type because of the high surface area of the starting glass powder. In many glasses the crystallization process is initiated primarily at the surface rather than in the bulk of the material. Thus high surface area appears to promote rapid crystallization in most of the known composition systems.
U.S. Pat. No. 4,501,602 describes a process for making high-silica glasses which includes the steps of forming a powder or soot of doped silica composition and then flame-sintering the soot at high temperatures to form clear or opalized (phase-separated or crystallized) glass. U.S. Pat. No. 4,718,929 describes methods for forming halide glasses based on BeF.sub.2 from vapor-deposited powders which likewise can produce either amorphous (non-crystalline) or polycrystalline metal halide products by high temperature sintering.
U.S. Pat. No. 4,537,864 discloses halide glasses in the Cd-Li-Al-Pb fluoride systems which, in the preferred composition regime, are sufficiently stable to be formed by casting to clear, crystal-free products up to about a centimeter in thickness. U.S. Pat. No. 4,323,654 discloses alkali phosphate glasses having sufficient stability to be formed by casting, while U.S. Pat. No. 4,362,819 discloses alkali fluoroaluminophosphate glasses for optical applications which can be cast into preforms and reshaped by molding.
As shown by the above patents, although relatively stable glasses have been developed in a variety of composition systems of marginal glass-forming stability, it will be readily appreciated that the compositional restraints imposed by the requirement for high glass stability limit the range of compositions which can be used. Even within the composition regimes described, compositions of less than optimum stability but exhibiting excellent optical, electrical or other chemical or physical properties have had no practical utility because of the difficulty of forming them into products of centimeter or greater thicknesses while avoiding devitrification. It will therefore be appreciated that many potentially useful borate, phosphate, halide, and even silicate glasses are known which can perhaps be formed by rapid quenching into glass articles of thin cross-section, but which are not available for convenient use in larger forms.
It is a principal object of the present invention to provide a method for making glass articles from unstable glasses which can be used to make products of substantial bulk or thickness notwithstanding the tendency of the glass toward devitrification below the liquidus.
It is a further object of the invention to provide a glass-forming procedure which utilizes reheating of crystal-free glass powders to form crystal-free glass products from glasses of low or marginal stability against devitrification.
Other objects and advantages of the invention will become apparent from the following description thereof.